The common method for producing sugar from sugar canes comprises squeezing juice from the stalk of sugar canes by milling and then boiling down the juice to obtain crude sugar.
Along with sugar component, however, the cane juice inevitably contains impurities such as the bark of crushed stalks, vascular bundles and small pieces of stalk pith as well as wax and resin. The wax and resin components are separated from the juice by precipitation followed by filtration, if necessary, with the addition of flocculating agents and filter aids. Other impurities such as the bark of crushed stalks, vascular bundles and small pieces of stalk pith are also eliminated from the juice together with the wax and resin components by the filtration. While crude sugar is produced by boiling down the resulting filtrate, the crude wax are produced from the filter cake (generally known as "cachaza"), which contains wax, resin, bark of crushed stalks, vascular bundles, small pieces of stalk pith in addition to precipitating agents and mud. This filter cake usually contains from 70 to 80 wt % of water, although it is solid in appearance.
In the first pressing, because the sugar cane is crushed or smashed, its filter cake obtained by filtration is a deformable solid substance which is an agglomerate of crushed substances containing water. The crushed pieces or broken pieces forming the agglomerate are in irregular forms or in fiber-like state. As it is an entangled agglomerate containing water, it is a lumpy mass maintaining its own form to some extent. It is not a viscoelastic substance in a true meaning, however, it seemingly exhibits the properties resembling those of a viscoelastic substance.
In Japanese Laid-Open Patent Publication No. 6-122892, there is proposed a method for continuously extracting crude wax using a hydrophobic organic solvent without any previous drying of the filter cake. In this method, the extraction is carried out while kneading the filter cake with the hydrophobic solvent and, as suitable apparatus for use in the method, transferring machines having heating and agitating means which are equipped with mixing elements such as screw, ribbon impellers or paddles, are proposed. Furthermore, it is described to be preferable that the filter cake and solvents are mixed together under shearing stress in order to break down the filter cake, which is partly in lumpy state, into small pieces and to disperse well in the solvents (ibid., page 4, column 5, para. 0017). However, because the substances to be treated are the mixture of the solvent in liquid state and the filter cake in solid agglomerate, the extraction can hardly be carried out by means of the above-mentioned simple transferring apparatus with a mixer such as the single shaft screw extruder as disclosed in the above patent gazette.
In U.S. Pat. No. 2,554,073, another method to produce wax from filter cake is disclosed in which a vertical continuous extracting column is used. In this method, counter-current extraction of filter cake and solvent is simply carried out by fingers and stirring arms which are turning round on each plate in the column. The water contained in filter cake is considered to exist in the crushed pieces and the spaces among the crushed pieces of the filter cake. When hydrophobic solvent is used, such water obstructs the solvent extraction. The obstruction of this kind cannot be avoided by the simple stirring operation. Meanwhile, if the vigorous stirring is done, the filter cake is dispersed into the solvent in the form of very fine pieces. The finely dispersed filter cake is not desirable because it is inconvenient for the subsequent transferring and other treatment and there is apprehension of the blocking of transfer pipe lines.
It has been found out in the studies carried out by the present inventors that, in the solvent extraction of the wax from the filter cake obtained by processing sugar cane, the wax is a main component of the extract in the early stage of the extraction, however, as the extraction proceeds, the extraction of other components like lignin from the sugar cane fiber tend to increase and the separation of such components from the wax component is very difficult even by utilizing a separation method such as recrystallization operation in a subsequent step. Therefore, it is necessary to control the retention time of the mixture in the extraction equipment in an adequate range in order to obtain crude wax with less lignin content.
From this point of view, a batchwise solvent extraction process has an advantage of offering comparatively narrow distribution of the extraction times. However, it is not suitable for the treatment of large quantities and it is essentially not adequate as an industrial production method.
The conventional methods are not always satisfactory in order to treat a large quantity of material and to avoid the dispersion of retention time in solvent extraction.
In view of the above described circumstances, the object of the present invention is therefore the provision of the extraction apparatus which permits the solvent extraction of crude wax from the solid residue as a raw material separated from the juice obtained by milling sugar cane without substantial dispersion in the extraction times and with high extraction efficiency.